Subaqueous Delta Research Articles

Oligocene-Miocene lacustrine rudite-dominated alluvial-fan delta, southwest Montana, U.S.A.

Facies continuum and facies profiles from outcrops aud drili cores in die Tertiar>' Medicine Lodge beds were analyzed in the Medicine Lodge and Horse Prairie basins in Southwest Montana, U.S.A. These betAs consist of interbedded conglomerates. sandstones, siltstones, mudstones, limestones, carbonaceous sitales, antA coals that malce up coarsening- and fining-upward megasequences. Tite study indicates titat die Medicine Lodge beds were deposited in freshwater lake alluvial-fan delta antA sitort-headed stream delta environments. The lake, whicit probabi>' occurred in an asymmetrical protobasin formed during Laramide deformation and ancestral to tite Medicine antA Horse Prairie basins, was initiali>' infilled by coal-forming, short-headed stream deltas antA littoral lacustrine carbonate platfonns alqng its steeply dipping margin. Ihis early stage of infilling was characterized by rapid basin subsidence, witich was accompanied by lake expansion, antA by subdued provenance tectonism developed west-soudiwest of tite protobasin. Ibe late stage of lake infilhing continued along tite steeply dipping margin of the protobasin where east-northeast flowing alluvial-fan delta complex formed an aggrading, subaerial alluvial fan antA a prograding, subaqueous delta front. Tite alluvialfan delta complex was bounded tu tite south by a lacustrine coastal plain that was drainedby coal-forming, short-headed streams deltas antA tu the immediate nortit by lacustrine littoral carbonate platforms that supported stromatolite Cuadernos de Geologia Iberica N.0 15, 241-278. Madrid, 1991. Edit. Universidad Complutense. R. Al. Flores & J. W. M'Gonigle 242 bioitenns. Titis stage of lake-basin infilling was marked by intense provenance uplifts and was accompanied by moderate basin subsidence antA local lake expansion with marked lake-level fluctuations. Climate did not significantly influence fluctuations of the lake level since tite lake remained fresitwater and open throughout infilling. However, microclimate (e.g., ram sitadows) may have controlled coal distribution in the Medicine Lodge basin part of tite protobasin. Humid-temperate climate is indicated by common presence of debris flows on the alluvial fan, which were triggered it>' intense rainfali. Titus, tite lake protobasin is a microcosm of clastic, carbonate and organic environments, itydrodynamics, provenance tectonism, climate, and basin subsidence titat greati>' influenced facies variations.

Sedimentology, structural evolution and tectonic setting of the late Precambrian Longmyndian Supergroup of the Welsh Borderland, UK

Abstract The Longmyndian Supergroup is an approximately 6500 m thick sedimentary sequence of late Precambrian age. Major unconformities within this sequence are lacking. The Longmyndian Supergroup can be divided into four facies associations: (1) turbidite, (2) subaqueous delta, (3) alluvial floodplain, and (4) braided alluvial. These are organized into a single, broadly upwards-coarsening, progradational sequence, with the turbidite association at the base and the braided alluvial association towards the top. The sediments are interpreted to have been deposited in a forearc, or possibly interarc, basin which lay to the north of a contemporaneous magmatic arc, of which the (now adjacent) Uriconian Volcanic Complex is considered to have been a part. The Longmyndian Supergroup has been deformed into a tight NNE-SSW trending syncline. Fault patterns and strain markers within the Longmyndian indicate that it was deformed during sinistral transpression. The adjacent Pontesford-Linley and Church Stretton fault systems are considered to be major strike-slip fault systems which were initiated at about the same time as the deformation of the Longmyndian Supergroup. Strike-slip movements along these fault systems resulted in the juxtaposition of the Uriconian Volcanic Complex with the Longmyndian Supergroup. Late Precambrian/early Cambrian deformation is indicated by some stratigraphic and radiometric age-dating evidence. Since there is evidence that the fault systems have suffered strike-slip displacements during the Palaeozoic it is difficult to be certain of the magnitude and style of the late Precambrian/early Cambrian deformation. However, the recognition of late Precambrian/early Cambrian sinistral transpressional deformation in northwest Wales suggests that the Precambrian of the Welsh Borderland was similarly deformed during the late Precambrian/early Cambrian.

Permian and Triassic Sedimentation in the Northeastern Brooks Range, Alaska: Deposition of the Sadlerochit Group (1)

Permian and Triassic rocks of the Sadlerochit Group crop out extensively in the northeastern Brooks Range adjacent to the coastal plain of the Arctic National Wildlife Refuge. Terrigenous clastic rocks of the Sadlerochit Group are separated from underlying carbonates of the Lisburne Group by a regional unconformity marking the Pennsylvanian-Permian systemic boundary. Lithologic and diagenetic patterns of rocks above and below the sub-Sadlerochit unconformity vary systematically with substantial paleotopography along the erosional surface. Within the Sadlerochit Group, nonmarine conglomerate of the basal Echooka Formation fills erosional channels and valleys and grades upsection into a retrogradational succession of tempestites deposited during episodic storm-surge in shal ow-marine environments. The overlying Ivishak Formation records an abrupt reversal of environmental migration pattern and the beginning of southerly progradation of a broad suite of deltaic environments. Depositional units in the Ivishak are strongly progradational and cyclic at a variety of scales, reflecting alternating episodes of progradation and transgression inherent in delta construction. The basal Kavik Member of the Ivishak Formation is an upward-coarsening depositional assemblage recording initial migration of prodelta environments and evolution of an extensive subaqueous delta platform. The overlying Ledge Sandstone Member is organized into three depositional assemblages recording the evolution of delta-fringe, distributary-channel, and crevasse-splay environments of the lower delta plain. The Fire Creek Siltstone Member is an aggradational and transgressive assemblage deposited in shallow-marine environments that reworked sediment originally deposited on the lower delta plain. The depositional assemblage records the final large-scale subsidence and drowning of the Ivishak delta platform. The most favorable reservoir strata are developed immediately above and below the sub-Sadlerochit unconformity and in distributary-mouth bar and distributary-channel deposits of the Ledge Sandstone Member.

Model of modern dynamic deposition in the east China Sea

Since the last rising of sea level, two branches of the Kuroshio, the Huanghai (Yellow Sea) coastal current (HCC; mainly cold water mass) and the Changjiang River outflow have controlled the modern dynamic deposition in the East China Sea. There are three depositing areas on the sea-bed under the above currents: a relict sand area un der the Taiwan Warm Current and the Huanghai Warm Current at the south-eastern area, the about 60 km2 round mud bank under the Huanghai Coastal Current at the northern area and the large subaqueous delta of mainly fine sand and silt under the Changjiang discharge flow in its estuary and the large narrow mud bank under the Zhejiang-Fujian Coastal Current, another round mud bank under the Changjiang discharge flow off Hangzhou Bay. The relict sand area has a coarsesand block under the Taiwan Warm Current bypassing Taiwan at the northern part of the island.

Dispersal and deposition of river sediments in coastal seas: Models from Asia and the tropics

The diverse mechanisms by which river-borne sediments are dispersed into coastal oceans and the associated patterns of deposition are considered for some tropical and Asian river mouth dispersal systems: the Huanghe (Yellow River), which enters the Bohai Gulf (China), the Purari River which enters the Gulf of Papua (Papua New Guinea) and the Jaba River, which enters Empress Augusta Bay (Bougainville, Papua New Guinea). These models contrast sharply with ‘conventional’ models such as that of the Mississippi, although in different respects. Extremely high suspended sediment concentrations off the Huanghe mouth cause sinking, gravity-driven plumes which produce rapid deposition very near the mouth; extremely rapid seaward growth of the subaqueous delta results. Although the average water discharge of the Purari exceeds that of the Huanghe, the average sediment discharge from the Purari is an order of magnitude less than that of the Huanghe. Suspended sediments transported via buoyant plumes from the Purari mouth are trapped inshore by the southeasterly trades and have their ultimate sink in the tidal estuaries to the west of the mouths rather than offshore. The Jaba is a small river with a very steep gradient and an extremely high bed load relative to water discharge. It has constructed a protruding and rapidly evolving delta. Literature on the Indonesian rivers Solo and Porong dispersal systems suggests that those systems may, at different times, be subject to processes similar to those which operate off the mouths of the Huanghe, Purari and Jaba although no single, direct analogies can be made.

Upper cretaceous to paleocene depositional sequences and sandstone petrography of southwestern Patagonia (Argentina and Chile)

Upper Cretaceous to Paleocene strata exposed along the Andean margin of Patagonia south of 50°S were deposited in a foreland basin and comprise a complex transition from continental to marine facies. Three unconformity-bounded sequences are observed within the sedimentary succession studied. Sequence 1 (upper Campanian) displays a rapid north-to-south transition from upper delta plain mudstones (Cerro Fortaleza Formation) through lower delta plain and subaqueous delta plain sandstones (La Anita Formation) to delta-slope and basinal turbidites (Alta Vista and Tres Pasos Formations). Sequence 2 (Maastrichtian-Paleocene) was initiated with braided river sediments (La Irene Formation), deposited over a Type II unconformity ( sensu Vail et al., 1984). These rocks are overlain by meandering fluvial sandstones and mudstones (Chorrillo Formation) that interfinger to the south with fossiliferous shallow marine sandstones (upper Cerro Cazador Formation). Sequence 3 (Paleocene) is composed of shallow marine conglomerates and crossbedded sandstones (Calafate and Cerro Dorotea Formations). These rocks are separated by an angular unconformity from subjacent units. Sandstone petrographic analysis indicates the presence of two main petrofacies within the rocks studied. A quartz-rich petrofacies, present to the north of the basin during late Campanian to early Maastrichtian time, was derived from continental crustal block and recycled orogen sources that were possibly exposed to the northwest of the basin. The second petrofacies (volcanic-rich petrofacies) was restricted to the south of the basin during the late Campanian to early Maastrichtian, but covered the entire area during the late Maastrichtian and Paleocene. These sediments were derived from a dissected magmatic arc located to the west of the study area. The presence of abundant tuffaceous intercalations, as well as fresh andesitic fragments, indicates contemporaneous volcanism near the Cretaceous/Tertiary boundary. The marked contrast in observed petrofacies may be a reflection of the major Andean segmentation at 49°S, at the present intersection of the Chile Rise with the South American coast.

Fan-Delta and Interdeltaic Shoreline Sediments of Middle Devonian Granite Wash and Keg River Clastics, Red Earth Field, North Alberta Basin, Canada: ABSTRACT

A detailed sedimentological investigation of over 4000 ft of core and 500 well logs of the Middle Devonian granite wash and Keg River clastics in the Red Earth field, North Alberta basin, Canada, has led to the recognition of a granite wash subaerial fan-delta system that is laterally continuous with a Keg River subaqueous delta component along an eastern shoreline of the ancestral Peace River arch. The subaerial fan delta includes alluvial fan facies, sheet wash and mud flows, and playa lakes. The subaqueous delta component includes lower shoreface, upper shoreface, beach-foreshore, eolian sand dunes, lagoon, washover sands, tidal channels and flats, and supratidal carbonates and anhydrites. Within this system, six mappable units are defined. A conceptual depositional model for the sequence depicts four main events. (1) Erosion of Peach River arch uplifted fault blocks, which produced coarse-grained fan-delta sediments in an adjacent fault-bounded margin. Subsequent fluvial reworking resulted in the deposition of thick, lenticular, wedge-shaped alluvial fans of granite wash. (2) Progradation of alluvial fans seaward into the Keg River Sea. (3) Transgression by Middle Devonian seas from the east, which reworked alluvial fans and led to deposition of discontinuous linear sand bodies represented by the Keg River regressivemore » shoreline sediments. (4) Restriction of the sea by the Presqu'ile barrier reef to the north, which deposited evaporites of the Muskeg Formation over the whole sequence. Modern analog to this fan-delta system is the coastal fans of the Gulf of Aqaba, Red Sea. Red Earth field contains over 27 million bbl of recoverable oil, related to a combination structural-stratigraphic trap.« less

Shelf sedimentation off the Ganges-Brahmaputra river system: Evidence for sediment bypassing to the Bengal fan

Research Article| December 01, 1989 Shelf sedimentation off the Ganges-Brahmaputra river system: Evidence for sediment bypassing to the Bengal fan Steven A. Kuehl; Steven A. Kuehl 1Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208 Search for other works by this author on: GSW Google Scholar Tina M. Hariu; Tina M. Hariu 1Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208 Search for other works by this author on: GSW Google Scholar Willard S. Moore Willard S. Moore 1Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208 Search for other works by this author on: GSW Google Scholar Geology (1989) 17 (12): 1132–1135. https://doi.org/10.1130/0091-7613(1989)017<1132:SSOTGB>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Steven A. Kuehl, Tina M. Hariu, Willard S. Moore; Shelf sedimentation off the Ganges-Brahmaputra river system: Evidence for sediment bypassing to the Bengal fan. Geology 1989;; 17 (12): 1132–1135. doi: https://doi.org/10.1130/0091-7613(1989)017<1132:SSOTGB>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The nature of shelf sedimentation seaward of the world's largest sediment dispersal system is examined by using sedimentological and geochronological techniques on a unique suite of sediment cores and grab samples. Sediments from the Ganges-Brahmaputra river system are currently accumulating on the shelf in water depths of less than about 80 m, forming a clinoform-like deposit similar to subaqueous deltas found off other major river systems. The highest sediment accumulation rates on the shelf occur near the head of the Swatch of No Ground, a major submarine canyon that indents the shelf west of the present river mouths. This observation, together with textural data, suggests that river sediments are transported seaward and westward and that the Swatch of No Ground is currently a major conduit for the transport of sediments from the Bengal shelf. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Slave River delta: geomorphology, sedimentology, and Holocene reconstruction

The Holocene Slave River delta (8300 km2) is a long (170 km), narrow (42 km average width) alluvial sand body, which extends north from the Slave River rapids at Fort Smith to Great Slave Lake, Northwest Territories. The delta is flanked by the Talston and Tethul rivers and Canadian Shield to the east and by the Little Buffalo River to the west. Wave-associated sedimentary structures in lithostratigraphic logs from river cutbanks indicate that the sandy delta was wave influenced. Most of the logs (34) consist of three facies: basal laminated mud (unknown thickness), interbedded mud and sand (2.5 m), and planar-tabular ripple sets interbedded with cross-laminated to flat-bedded sand (3.0–14.5 m).Eleven radiocarbon-dated wood samples from river cutbanks were used to reconstruct the delta paleoshoreface and to calculate the rate of progradation, which averaged 20.7 m/year from 8070 BP to the present. In the same period isostatic rebound of the delta region relative to the Liard River delta averaged 12 cm/km (a total rebound of 48 m). The data were calculated normal to the retreating Laurentide ice front.From the surface to depths of 59 m, the subaerial and subaqueous delta front exhibits barrier islands, lagoons, offshore bars or sand waves, tensional cracks, slumps and pressure ridges. The barriers and offshore bars consist of medium grain-sized sand, whereas the slumps and pressure ridges are interpreted as mud.

Dynamic changes and processes in the Mississippi River delta

Research in the modern delta of the Mississippi River has revealed short-term changes and processes that are of significant magnitude. Deltaic lobes, each lobe covering an area of 30,000 sq km and having an average thickness of 35 km, switch sites of deposition on an average of every 1,500 yr. Through short periods of geologic time, this process results in a relatively thick accumulation of stacked deltaic cycles covering extremely large areas. Within a single delta lobe, and operating on an even higher frequency, are bay fills and overbank splays. Bay fills, having areas of 250 sq km and thickness of 15 m, require only 150 yr to accumulate. Four major events have taken place in the modern Balize delta since 1838. Overbank splays are much smaller, covering areas of less than 2 sq km and having thicknesses of 3 m, but are associated with high floods on the river. At the river mouth, continued progradation of the distributary channel can form distributary mouth sand bodies that have dimensions of 17 km long, 8 km wide, and a thickness of 80 m in a period of only 200 yr. Differential sedimentary loading at the river mouth results in formation of diapirs that display vertical movements in excess of 100 m in a period of 20 yr. On the subaqueous delta platform, sediment instabilities operate nearly continuously, mass-moving large quantities of shallow-water deposits to deeper-water environments via arcuate rotational slides and mudflow gullies and depositional lobes. All of these changes and processes operate at differing spatial and temporal scales, but all result in deposition of large volumes of sediment over extremely short periods of time.

Facies Development of Atchafalaya Delta, Louisiana: A Modern Bayhead Delta

Early studies of Atchafalaya delta focused on large-scale characteristics of accretion in Atchafalaya Bay. This study concentrates on sedimentation processes and stratigraphic as well as plan-view facies development in a rapidly prograding bayhead delta. Since initiation of subaqueous delta growth (early 1950s), the bay has become shallower with accumulation of progressively coarser grained sediment. Stratigraphic relationships indicate a buildup from the pre-1950 bay bottom through prodelta, distal bar, distributary-mouth bar, levee, back-bar algal flat, and channel-fill deposits. At the present stage of delta development, two distinct processes of subaerial growth are recognizable. Initial growth relates to channel extension, bifurcation, and development of sinuous overbank channels. A later, more mature response of lobe fusion and upstream lobe growth becomes dominant as progradation through bifurcation diminishes in older areas of the delta. Atchafalaya delta growth to 1982 serves as a well-documented sedimentary example for understanding bayhead deltas, which commonly form in protected environments.

Microfabric Study of Fine-grained Sediments: Observations from the Amazon Subaqueous Delta

ABSTRACT Sedimentary microfabric (millimeter-scale) is examined in thin sections of fine-grained sediments from the Amazon subaqueous delta. Coarse silt (4-6 ) laminae are abundant in the proximal portion of the Amazon subaqueous delta and are composed of numerous grain types, including quartz, feldspar, heavy minerals, wood fragments, and fecal pellets. In distal areas, the coarse silt fraction is absent (because of preferential accumulation of coarser material nearer to the source), and laminae generally are thin ( 0.1 mm) and are composed of fine quartz silt. Most silt laminae are observed within a matrix of platy minerals, and basal contacts are generally sharp, indicating scouring of th sea bed during formation of the laminae. Silt laminae form as a result of changes in sediment supplied from suspension and from particle-sorting processes operating near and on the seabed. Extinction phenomena of clay minerals and mica in thin sections reveal a unique category of sedimentary structures that is not observable using other (e.g., radiographic) techniques. Two extinction patterns (termed plasmic fabric) are observed: 1) a layered fabric that consists of horizontal layers of relatively coarse, platy minerals, which are interlayered (0.1 mm vertical scale) with unoriented, finer platy minerals, and 2) a uniform fabric with constituent minerals uniformly oriented parallel to bedding (on vertical scales of at least 1 mm). The fabric types represent primary (physical) sedimentary structures; however, the mechanisms for their formation are uncertain. The effect of bioturbation on microfabric is observed in thin sections where burrows disrupt silt laminae and erase evidence of the plasmic fabric. In the seaward portion of the Amazon subaqueous delta, the extent to which bioturbation alters physical structures depends on the ratio of biological mixing rate to sediment accumulation rate. Ratios greater than about ten indicate nearly complete homogenization of the sediment. The nature and distribution of microfabric in the Amazon subaqueous delta provide important information for interpretation of ancient mudrocks.

A JURASSIC DELTA IN THE EASTERN ALBORZ, NE IRAN

A clastic dominated Lower‐Middle Jurassic sequence, known as the Shemshak Formation, is extensively exposed in the Eastern Alborz area, NE Iran. The unit represents a deltaic complex sandwiched between carbonate rocks. Sedimentological study of the formation outcrops has revealed that it was deposited in environments ranging from subaerial to subaqueous delta plains.Potential source, reservoir and cap rocks are present within the Shemshak Formation; under favourable conditions, generation and accumulation of hydrocarbons in significant quantities can be envisaged. It is suggested that the Shemshak delta, buried under the thick Neogene sediments in the Eastern Alborz area, is a highly prospective exploration target with potential for a multipay section.

The subaqueous delta of the modern Huanghe (Yellow River)

The subaqueous delta of the Huanghe (Yellow River) has been studied using high-resolution acoustic systems. There are many subtle variations in sea floor morphology and sediment geometries; smooth, featureless areas are rare. The main components of the subaqueous delta include broad, shallow channels; moderately disturbed areas with near-surface cut and fill structures; heavily disturbed areas with sea floor depressions, pits, and gullies; and a smooth, gently sloping distal delta apron or “rise.” These features are not directly related to sediment settling from dilute surface plumes but are due to gravity-driven hyperpycnal underflows, submarine mass movements, and silt flows.

Sedimentary framework of the modern Huanghe (Yellow River) delta

The geometry, stratigraphy, and structure of recently deposited Huanghe (Yellow River) Delta sediments were examined by high resolution subbottom profiles and medium-penetration boomer profiles. The results indicate that the active (post-1976) subaqueous delta advances as a single thin localized lobe with a maximum thickness of only 15 m. Calculations of sediment volumes indicate that 90% or more of the sediment supplied by the Huanghe remains within 30 km of the mouth. Sediment on the delta platform near the mouth is fine sand; elsewhere silts and clays prevail.[/p]

The deltaic nature of Amazon shelf sedimentation

Despite the annual discharge of more than a billion tons of sediment by the Amazon River, the sedimentary environment near the river mouth has little subaerial expression and thus does not meet the classic definition of a delta. The river mouth, however, is not an estuary, either. These observations raise a major question as to what type of sedimentary environment the Amazon river mouth represents. Seismic stratigraphy has been examined on the continental shelf at the mouth of the Amazon River using high-frequency (3.5-kHz) seismic records from about 6,000 km of ship track. These records demonstrate three regions. (1) The Amazon River has built a subaqueous feature which stretches for hundreds of kilometres offshore and alongshore from its mouth. The feature is prograding seaward and accreting upward, and it contains fine-scale stratification typical of classic deltas. The feature forming at the mouth of the Amazon is a subaqueous delta; it differs from classic deltas primarily in its lack of subaerial expression. Subaqueous deltas, such as the Amazon, represent the general case of a major river entering an energetic oceanic regime.

Nature of sediment accumulation on the Amazon continental shelf

Sediment accumulation on the Brazilian continental shelf near the Amazon River is investigated using radiochemical (e.g. 210Pb, 14C) techniques to provide a better understanding of this major dispersal system of fine-grained sediment. 210Pb profiles from 57 cores collected during 1983 reveal the distribution of modern (100-y time scale) accumulation rates on the Amazon subaqueous delta. Accumulation rates increase from <0.1 cm y 1 (0.1 g cm 2 y 1) nearshore, to rates as high as ∼ 10 cm y −1 (6.9 g cm 2 y −1) on the outer topset and the foreset regions (30–50 m water depth). Reduced upward accretion nearshore (<15 m water depth), which is reflected in the limited subaerial expression of the Amazon delta, probably results from the intense activity of surface waves and tidal currents. A thick (as much as 2 m) depth). This layer probably is reworked by waves and currents, and most of the sediment is eventually transported to other parts of the dispersal system. 14C dating of an anomalous area of relict (age >100 y) sediment in the northwestern portion of the subaqueous delta indicates that this sediment was deposited <1000 y ago. The absence of modern sediment in this area is not understood. A sediment budget for the Amazon shelf indicates that 6.3 ± 2.0 × 10 8 tons of sediment accumulate annually. Much of the remainder of Amazon River sediment (∼6 × 10 8tons y −1) probably is transported northwestward beyond the Brazilian shelf and/or is accumulating landward of the shelf as coastal accretion.

High-resolution seismic stratigraphy and its sedimentological interpretation on the Amazon continental shelf

Seismic profiles (3.5 kHz) from over 6000 km of shiptrack and piston cores from 26 stations were examined to evaluate the seismic-stratigraphic framework of the Amazon subaqueous delta. Acoustic reflectors in the seismic profiles were related to sedimentological and acoustic properties in piston-core sediment. Grain size in cores from the Amazon shelf correlates well with seismic velocity and saturated bulk density, and thus with changes in acoustic impedance. Acoustic reflectors on the Amazon shelf generally correspond to coarser layers in the seabed. The observed acoustic reflectors can be attributed to both depositional and erosional processes. Depositional reflectors, which correspond to stratal surfaces, reveal typical stratification of the subaqueous delta. In a seaward direction, topset strata (<30 m water depth) dip gently (<1:1500) and diverge (∼1.5m/50km); foreset strata (30–70 m water depth) dip steeply (>1:1500) and converge abruptly (∼1.5m/5km); and bottomset strata (>70 m water depth) dip gently (<1:1500) and downlap onto the transgressive sand layer. Erosional reflectors correspond to hiatuses or to unconformities which truncate tops of seismic sequences composed of depositional reflectors. A stratum corresponding to an erosional reflector is actively forming over an area of 6 × 10 3km 2 seaward of the river mouth, and is thick (>2 m thickness) and coarse (5.1 phi, mean grain size) as compared to depositional reflectors. This erosional reflector overlies two seismic sequences of top-truncated foreset reflectors. North of the river mouth, an acoustically transparent layer, covering 1.5 × 10 4km 2 and representing 100–200 years of sediment accumulation, overlies truncated relict mud <1000 years old. The presence of truncated strata and erosional reflectors illustrates the spatial and temporal variability of sedimentary conditions on the Amazon shelf, and the difficulty of extrapolating modern (i.e. surficial) sedimentation patterns into the past.

Distribution of sedimentary structures in the Amazon subaqueous delta

The nature and spatial relationships of sedimentary structures in the Amazon subaqueous delta are determined from radiographs of widely distributed sediment cores, and are contrasted with observations of sedimentary structures from other major deltas. Five distinct facies are identified in modern (accumulated within the past 100 y) Amazon sediments, including: (a) physically stratified sand, (b) interbedded mud and sand, (c) proximal-shelf sandy silt, (d) faintly laminated mud, and (e) mottled mud. In the outer topset and foreset regions of the delta, organic-rich laminae are observed in cores from several of these facies. Physical sedimentary structures are preserved in the landward portion of the topset region (<30 m water depth) because deep scouring and subsequent redeposition places the structures below the depth of effective biological mixing. Physical reworking is reduced in the outer delta (30–70 m water depth), and the preservation of primary structures depends on the relative magnitudes of biological mixing rate and sediment accumulation rate. The amount of sand decreases along the dispersal system to the northwest, and interbedded mud and sand displays a northwestward gradation into faintly laminated mud. Longer-term (>100 years) variations of sedimentary structures are observed in piston cores. Thick (>1 m) sandy-silt beds are present in proximal areas below 3 m depth in the seabed, and could have formed as a result of lateral migration of the proximal-shelf sandy-silt facies.

Development of the Changjiang estuary and its submerged delta

Since the postglacial transgression, the Changjiang estuary has undergone a progressive progradation seaward of shoals and tidal flats fringing the south bank, merging of linear sandbanks with the north bank, successive filling and narrowing of the estuarine embayment, and a resultant seaward migration of the river mouth. Knowledge of the development and evolution of the Changjiang estuary and its subaqueous delta are critical in selecting navigational channels, managing coastal resources and fisheries, as well as reclaiming coastal areas for cultivation.